A thermal method for material analysis is for example differential thermal analysis (DTA) from the group of methods for thermal analysis. DTA is based on a characteristic energy conversion during the phase transition and enables the qualitative analysis. Temperatures of the sample and of a selected reference substance are each measured and compared in a crucible in a symmetrical measurement chamber. The reference substance is selected such that it does not exhibit any phase transitions in the temperature range to be investigated. A constant energy supply takes place by means of a furnace. The temperatures beneath both crucibles are measured by a temperature sensor and the occurring difference is recorded. Only with phase transitions does such a temperature difference occur, from the curve shape whereof conclusions can then be drawn as to the composition of the sample. Frequent areas of application of DTA are the investigation of mineral substances, such as for example clinker phase formation in cement raw meal, the detection of the heat of reaction in the combustion of organic substances and the characterisation of plastics.
Dynamic differential calorimetry (engl. differential scanning calorimetry, DSC) has been developed further from DTA. Instead of directly recording the temperature difference between the two crucibles as a function of the supplied energy and the temperature of the reference substance as in the case of DTA, the heat flow difference is determined therefrom in the case of DSC. Dynamic differential calorimeters (DDK, engl. dynamic differential scanning calorimeters, DDSC) are used for the analysis of polymers, pharmaceutical materials, textiles, metals, ceramics and other organic and inorganic materials. Various material properties such as phase transition temperatures, specific heats, melting and solidification temperatures, etc. can be derived from the measured magnitudes. The method of dynamic differential calorimetry is established and standardised worldwide (ISO 11357, DIN 52765, ASTM E 967, ASTM 968 or ASTM D 3418). A distinction is made between power-compensating DSC and heat flow DSC.
In the case of these measuring devices, two ceramic or metallic crucibles are typically used to receive a sample and a reference. In the case of a power-compensating DSC, the two crucibles are inserted separately into two small furnaces, which are often equipped with resistance heating such as for example a platinum heating coil. Apart from this, there are various possibilities for cooling such as for example liquid nitrogen cooling, compressed air cooling, mechanical cooling systems and so forth. Both crucibles are subjected to the same temperature program. The difference in the electrical power that is required to keep a temperature difference between the two crucibles constant, typically at zero, is measured. PT100 resistance thermometers, welded thermocouples or thermopiles are usually used nowadays in practice as temperature measuring devices.
With heat flow DSCs, on the other hand, there is only one furnace, which is usually heated with the aid of resistance heaters such as for example jacket heating conductors. For the cooling, the same methods are available as for power-compensating. A sensor with two support surfaces or defined positions for the sample and the reference is installed in the furnace. The support surfaces can for example be integrated into a disc or be located on a cylindrical elevation. They are contacted with temperature measuring devices (PT100 resistance thermometers, thermocouples or thermopiles) and are each provided with a corresponding crucible during the measurements. The heat flow difference or the temperature difference between the two positions is measured directly using the temperature measuring devices.
The temperature difference can be converted into a heat flow difference if the DSC device has undergone a calibration. This can take place with reference materials, the relevant thermal properties whereof (e.g. temperature onset and enthalpy of phase transitions, specific heat capacity as a function of the temperature) are precisely known. A very important reference material of DSC is indium.
The accuracy of the measurement of the heat flow depends on how well the measurement signal can be reproduced when the actual heat flow of a sample is the same. An essential pre-requisite for good reproducibility is that the overall thermal resistance along the heat flow path between the sample and the reference remains as far as possible identical in successive measurements. The overall thermal resistance results from the sum of the individual resistances along the heat flow path. The latter are determined essentially by the thermal conductivities of the materials used, the geometry of the components and the contact resistances at interfaces (e.g. crucible/sensor).
Heat flows which take place by heat conduction through the surrounding gas, convection and radiation must not however be overlooked. With regard to the heat conduction through the surrounding gas, this can be detected in the case of heat flow DSC by measurements of the melting process of indium in different gas atmospheres. If the melting of indium is measured once under helium and then the same sample under the same measurement conditions under argon, the integral of the melting peak in the temperature difference curve for the measurement under argon is greater than under helium. The reason for this is the markedly lower thermal conductivity of argon compared to helium. In the case of the measurement under helium, therefore, a greater part of the heat flow between the indium sample and the reference flows via the gas than under argon. This proportion of the heat flow, however, is virtually undetected by the temperature measuring devices and the temperature difference measured by the temperature measuring devices thus produces a smaller integral in the case of helium. In practice, this phenomenon is taken into account by calibrations depending on the type of gas.
Crucibles of different shape made of different materials are used for the measurements depending on the application and its particular requirements. The mass of the crucibles should be as small as possible, the heat conduction should be good and, for industrial use, the price should be as low as possible. A frequently used material, therefore, is aluminium. The wall and base thicknesses of the crucibles lie in the range of a few tenths of a millimeter, the filling volume between a few tens and a few hundred microliters. In order that the thermal resistance is maintained and therefore the accuracy of the heat flow measurement is not impaired, the actual contact surfaces between the crucible base and the support surface must not vary for different crucibles. The problem here, in particular, is that the crucible bases can deviate from an ideally flat shape in an uncontrolled and non-reproducible manner on account of the small material thickness. This may be caused by production, but also by deformation during handling. A curvature of the crucible base outwards becomes evident in a particularly disadvantageous manner.
The individual components of the sensor, i.e. in particular the support surfaces and temperature measuring devices, are fixedly connected to one another, so that the resistance for the heat flow in this region does not change or changes only negligibly over a large number of measurements. Measurable changes can be compensated for by a recalibration. On the other hand, the thermal contact between the sample and the crucible and between the crucible and the sensor are more critical, because easy separability is usually desired at these points for practical reasons of handling.
On account of the large number of different sample shapes, the thermal contact between the sample and the crucible possibly has to be adapted for each individual sample. In this regard, there is relevant literature which deals with various possibilities for preparing samples (e.g. Achim Frick, Claudia Stern: DSC-Prüfung in der Anwendung. Munich and Vienna: Carl Hanser Verlag, 2006).
U.S. Pat. No. 7,470,057 and patent application DE 11 2007 001 888 disclose a sensor, wherein the support surfaces for the sample and the reference lie on the upper side of a sample platform and a reference platform. The sample platform and the reference platform are connected using diffusion welding to a cylindrical thin-walled element for the sample and to a cylindrical thin-walled element for the reference. The platforms for the sample and the reference are made of the one alloy of a thermocouple pair (alloy A) and the respectively associated cylindrical thin-walled elements are made of the other alloy of this thermocouple pair (alloy B). A base, which is made of the same alloy B as the cylindrical thin-walled elements, connects the latter together. A temperature difference can be measured via two wires made of alloy A, which are fixed to the undersides of the sample platform and the reference platform. It involves the difference between the mean temperature at the interface between the platform and the cylindrical thin-walled element on the reference side and the mean temperature at the interface between the platform and the cylindrical thin-walled element on the sample side. These interfaces lie outside the contact areas between the crucible and the platform. This is thus intended to ensure that the measured temperature differences remain independent of variations in the contact resistance, because the entire heat must flow via these interfaces according to the applicant's embodiments.
It is not taken into account here that, in the presence of a raised contact resistance between the crucible and the sensor, the heat flow proportions via radiation, heat conduction in the surrounding gas and convection increase in relative terms. The consequence of this is that the heat no longer flows to the same extent via the interface, which of course is at a certain distance from the crucible, and ultimately a smaller temperature difference, i.e. a weaker measurement signal, is built up.
In order that the thermal resistance is maintained and therefore the accuracy of the heat flow measurement is not impaired, the actual contact surfaces between the crucible base and the support surface must not vary for the various crucibles. The problem here, in particular, is that the crucible bases can deviate from an ideally flat shape in an uncontrolled and non-reproducible manner on account of the small material thickness. This may be caused by production, but also by deformation during handling. A curvature of the crucible base outwards becomes evident in a particularly disadvantageous manner for the reproducibility of the measurement results.
The problem underlying the invention, therefore, is to make available a method and a device for thermal material analysis and a sample holder with which the reproducibility of thermoanalytical measurements can be improved. Furthermore, it is a problem of the invention to provide a particularly effective production method for a holding device of a thermal analysis device.